Overview
This lecture explains the structure of DNA and details the step-by-step process of DNA replication, emphasizing the role of key enzymes.
DNA Structure
- DNA consists of two strands twisted in a double helix.
- Each strand is composed of four chemical bases: adenine (A), cytosine (C), guanine (G), and thymine (T).
- The strands are complementary: A pairs with T, and C pairs with G.
- Each DNA strand has a 5' end and a 3' end, and the strands run in opposite directions.
DNA Replication Process
- Replication starts by separating the two DNA strands using the enzyme helicase, forming a replication fork.
- Each separated strand serves as a template for a new strand.
- Primase creates a short RNA primer to mark the starting point for new DNA synthesis.
- DNA polymerase binds to the primer and adds new DNA bases from 5' to 3'.
- The leading strand is made continuously in the 5' to 3' direction.
- The lagging strand is made discontinuously in small segments called Okazaki fragments, each starting with an RNA primer.
- DNA polymerase fills in bases for each Okazaki fragment in the 5' to 3' direction.
Final Steps in Replication
- Exonuclease removes all RNA primers from both DNA strands.
- Another DNA polymerase fills in the gaps left by the removed primers with DNA.
- DNA ligase seals the fragments to create continuous double strands.
Key Terms & Definitions
- Double Helix — the twisted ladder shape of DNA.
- Complementary Strands — strands where each base pairs with its specific partner (A with T, C with G).
- Replication Fork — the area where DNA strands are separated for replication.
- Primer — a short RNA segment that initiates DNA synthesis.
- Leading Strand — the DNA strand synthesized continuously.
- Lagging Strand — the DNA strand synthesized in fragments.
- Okazaki Fragments — short DNA segments made on the lagging strand.
- Helicase — enzyme that unwinds and unzips DNA strands.
- Primase — enzyme that makes RNA primers.
- DNA Polymerase — enzyme that builds new DNA strands.
- Exonuclease — enzyme that removes RNA primers.
- DNA Ligase — enzyme that joins DNA fragments.
- Semi-conservative Replication — each new DNA molecule has one old and one new strand.
Action Items / Next Steps
- Review diagrams of the DNA replication process.
- Memorize the key enzymes and their roles in DNA replication.